Such a two-dimensional measuring data structure can be found, for example, in analytic measurement systems, in particular in the field of UV/VIS absorption measurement technology. In this case, diode array detectors (DAD's) are used. Diode array detectors are constantly gaining importance because, firstly, the UV/VIS absorption measurement procedure represents the most significant of the various analytic detection techniques and, secondly, DAD's generate online signals corresponding to the wavelength (spectra) and to time (chromatograms). UV/VIS absorption measurement technology is a good compromise between high sensitivity, high selectivity, and linear measuring range over several orders of magnitude. From the two-dimensional data structure, information about the identity (spectra) of the substances to be analyzed and about the amount of the sample substance (concentration) may be derived.
In traditional procedures, the concentration course of the substance to be studied is measured along the time axis for selected wavelengths after the substances of a sample mixture have been separated with respect to time, using a chromatographic analyzing technique. The measurement wavelength in this case is selected for optimum measuring conditions according to the absorbance of the respective substance. This means that, at lower concentrations, a wavelength with high extinction is used while a wavelength with low extinction is rather used for high concentrations. A wavelength with medium extinction may be used as a compromise. The respective substance amount may be determined from the concentration course by integration over time with appropriate gauging.
The spectra are determined from the data points along the wavelength axis. The spectra exhibit a substance-specific spectral characteristic that is used to identify the measured substance. The identification is done by comparison with spectra of known substances. Often information on the completeness of the separation is also derived from the measuring data measured. This is done by comparing the spectrum at the start of a chromatographic peak to the spectrum of the peak maximum or the spectrum at the end of a peak. If the spectra only differ by a factor, it is assumed that the peak at hand was completely separated during the analysis. The substance amount of the corresponding peak can then be determined correctly.
For measurement systems using DAD's, the measured data is subject to a number of undesired influence quantities that can affect the accuracy and reliability of the measuring results. Such influence quantities are either due to the shortcoming of the system components used and to the variations of environmental conditions or due to the selection of the analysis parameters according to the measurement method.
Such influence quantities include the intensity stability and the spatial expansion of the plasma of the light source, alterations of the optical projection due to thermal expansion of the system, changes to the optical ray trajectory due to the solvent or flow-related processes in the flux cell (index of refraction, flow profile, etc.), influencing of the measurement system due to disturbances by other modules (e.g. pressure surges, pump ripples, etc.), disturbances affecting the measurement system from the outside and altering the signals, degradation or alteration effects of system components used, thermal heating or transient behavior, random noise or pattern noise, and similar disturbances.
Regarding the Stability of the Light Source:
Spectrophotometric measurement systems often use gas-discharge lamps (e.g. deuterium lamps) as light sources. After the ignition process, a radiant plasma is created between the anode and the cathode of such a light source. The intensity and the spatial expansion of the plasma are subject to variations in time, which are due to the stability of the power supply and the burning properties due to the technical layout of the individual lamp. Such variations in time cause errors in the measuring data.
Regarding the Thermal Expansion of the Optical Equipment:
Due to fluctuations of the environmental conditions and because of the thermal expansion of the materials used, the imaging properties of the optical equipment are affected. This also causes errors in the measuring results.
Regarding the Disturbance of the Optical Ray Trajectory Due to the Solvent and Flow-Related Processes in the Flux Cell:
During the analysis, the substances to be measured, which were separated in the column through chromatographic separation and dissolved in the mobile phase, are led through the flux cell. During the absorption measurement, the changes in intensity and in the spectral composition of the light due to the processes in the flux cell are measured. From this, the absorption course and, based on the Lambert-Beer law, the concentration course of the substances to be studied is determined. Using appropriate procedures, the substance amount of the sample to be studied can be calculated from these values.
Depending on the analysis method, the composition of the mobile phase is often purposely changed during the analysis in order to optimize the separation of the substances. This means that during the analysis, the optical properties of the mobile phase are also changing due to the dynamic change in index of refraction and absorption characteristics, thus sometimes considerably affecting the measuring results. The value for the static index of refraction of the mobile phase is also affected by temperature fluctuations. An addition, pressure fluctuations, caused by the solvent pumps, adversely affect the optical properties.
Regarding the Disturbance of the Results by Random Noise or Pattern Noise:
Random noise superimposed over the measuring values is caused by stochastic processes occurring during the emission of light by gas discharge and during the detection of the photons by a photo sensor. In addition, there are noise terms caused by the circuits of the electronic signal processing system or by influences of the surroundings. The characteristics of such noise terms can be subject to a statistical distribution function or be characterized by system-specific patterns.
The noise adversely affects the accuracy of the measuring results and reduces the limit of detectability of the measurement system. Up to now, this has been met with filtering the measuring data in the time and wavelength dimensions. However, this is done at the expense of the time or spectral resolution of the signals.
It has always been a challenge in the design of measurement devices to minimize the effects of influence quantities in order to avoid significantly distorting the measuring results. In the past, some complex approaches have been realized to fulfill the requirements. Corresponding to the complexity of the approaches, different results have been achieved.
In order to fulfill the highest possible demands with respect to accuracy and precision of the measuring results, so-called two-beam systems have been designed in the past in order to suppress the effects of the intensity instability and the spatial expansion of the light source. The optical light beams are split into a reference path and an analysis path. Assuming that the optical imaging properties are identical for both paths, disturbances may be eliminated by a calculation of ratio. On closer examination, however, it becomes evident that two optical paths with identical properties can hardly be realized. Difficulties occur because the analysis path contains the actual flux cell while a model of such a flux cell is contained in the reference path.
Changes in the optical properties due to disturbances inside the flux cell (index of refraction, solvent absorption, streaks, etc.) or due to the flux cell itself can therefore not be detected by reference creation.
When using traditional approaches, the problems resulting from the dependence of the index of refraction on different factors can only be improved through complicated optical constructions and usually only at the expense of the light flux rate. This adversely affects the limits of detectability.
Furthermore, it should be noted that because of the lower light flux rate due to the splitting of the optical ray trajectory on one hand and due to the additional noise source due to the reference path on the other hand, the noise contribution of the measurement system is made considerably worse. Not only are the costs increased compared to simpler designed measurement systems, but a number of unfavorable compromises have to be accepted as well.
In the meantime, technology has reached a state wherein hardly any improvements can be achieved through constructional measures with acceptable cost and effort.
Past endeavors to improve the accuracy and limit of detectability with the help of suitable algorithms were concentrated on filter algorithms that reduce the statistical uncertainty, i.e. the noise. However, the disadvantage of filtering the data is the adverse effect on the time and/or spectral resolution of the signals. Further efforts also become meaningless when the systematic error contributions are dominating and reducing the accuracy.